Warning! Mayor spoilers. Note: Do the neutral run before reading this.
Glyde, Second Location
First - you must go there before fighting with Papyrus. Where? Remember place with X, O and ^ puzzle with slippery ice? Go there and go pass the ice bridge. Go down, next go left < and you will find Sans. Still go left. When you will find cave, go there and go up. Now press v, <, > and ^ quickly and repeat this for some time. A boss will appear!
So Sorry, Fourth Location
Working only on pacifist run.
This is little bit harder. First, you must make a neutral ending. Load your save file and go back to Hotland. Save in this place, when you must fight with Royal Guards. Turn off the game and change date to 10th october, 08:00 PM/20:00. Turn on the game and load save file Go right and you should hold ^ and > buttons. When you find secret glass bridge, go up and you will find the boss.
How to make snowman happy. Go to snowman location, take 1 piece, and do a neutral ending.